1. Field of the Invention
This invention relates to a ceramic heating element to be used as a heating element in an infrared ray source as well as an electric furnace.
2. Description of Related Art
A ceramic resistance heating element as disclosed in Japanese Patent Application Laid-Open No. 296,833/1993 is shown as a conventional example in FIG. 1.
In FIG. 1, a ceramic heating element 10 is produced, for example, by extrusion of molybdenum disilicide prepared using water as a binder, winding up the resulting molybdenum disilicide of small diameter in a coiled form, drying it, and firing it in a vacuum furnace to form a heating element. Lead wires 12a and 12b are connected to both ends of the coiled ceramic wire (heating element) 10. When electricity flows through the coiled ceramic wire 10 via the lead wires 12a and 12b, the coiled ceramic wire 10 generates heat.
The coiled ceramic wire 10 is disposed in a heat resistant ceramic (e.g., alumina) tube 16 provided with an infrared ray irradiation window 14. The joints between the ceramic wire 10 and the lead wires 12a and 12b are fixed in the ceramic tube 16 by means of a heat-resistant adhesive 18a and 18b. Accordingly, when electricity flows through the coiled ceramic wire 10 via the lead wires 12a and 12b, infrared rays emanated from the coiled ceramic wire 10 come out via the irradiation window 14.
Meanwhile, MoSi.sub.2 is utilized in the form of a heating element for use in a high-temperature furnace (e.g., at 1,750.degree. C.) in the air since it exhibits an excellent oxidation resistance by virtue of its capability of forming a protective film of silicon dioxide. Accordingly, MoSi.sub.2 has attracted attention as a high temperature structural material for use in a gas turbine member and the likes and has been evaluated as such a material.
Since MoSi.sub.2 involves problems of brittleness at low temperatures and low strength at high temperatures, however, a difficulty has been experienced in materializing therefrom a heating element. Further, since the resistivity of MoSi.sub.2 as a property of the material is as low as 0.0003 (.OMEGA.cm), a large amount of electric current is necessary for heat build-up of MoSi.sub.2 to a high temperature (1,300.degree. C.). This involves the problem of increased electric power consumption. Furthermore, the large amount of electric current to heat the MoSi.sub.2 to a high temperature develops an electromagnetic force. Attractive and repulsive forces developed by the electromagnetic force as well as thermal stress developed by the heat build-up act on the heat-generating portion of the heating element, so that creep deformation may occur leading to rupture thereof. Thus, the heating element made of MoSi.sub.2 involves the drawback of a short life span.
As for electric power consumption, various methods of increasing the apparent value of resistance of MoSi.sub.2 have been proposed, a representative one of which is a method wherein a small-diameter wire is formed from MoSi.sub.2.
Since a general method of producing an MOSi.sub.2 heating element comprises firing of an MoSi.sub.2 powder, however, not only formation therefrom of a small-diameter wire but also miniaturization of the heating element are out of the bounds of possibility. A primary cause of this is that the firing method itself is not suitable for formation of a small-diameter wire. Accordingly, even when formation of a small-diameter wire is attempted, the diameter of the resulting wire becomes large to some extent whereby a high value of resistance thereof cannot be secured, thus requiring a large amount of electric current.
Even if mechanical working of an MoSi.sub.2 wire is effected after firing thereof, the ceramic is so brittle in itself that the wire cannot be worked to such a small diameter as to be usable as a light source. Thus, it has been considered very difficult to develop a high-temperature MoSi.sub.2 heating element usable as an infrared ray source element at a temperature of at least 1,000.degree. C.
An invention according to which molybdenum disilicide is combined with a second phase of a boride in order to improve the toughness and high-temperature strength of molybdenum disilicide for use as a high-temperature structural material or as a heating element is disclosed in Japanese Patent Application-Laid-Open No. 195,061/1985. Another invention according to which molybdenum disilicide is combined with a second phase of molybdenum and tungsten boride in order to endow molybdenum disilicide with a sufficient oxidation resistance even at a temperature of at least 1,600 .degree. C. in addition to improve mechanical properties is disclosed in Japanese Patent Application Laid-Open No. 115,876/1989.
However, the ceramic composite materials according to the above-mentioned inventions cannot be said to be satisfactory for use as an high temperature infrared ray source element as mentioned above.